Antenna systems can employ a phased array of antenna elements to generate a steerable mainbeam with spatial sidelobes. Controlling the gain and phase of individual antenna elements shapes and steers the beam in a desired direction. Without good pattern measurements for each antenna element the beam forming may produce undesirably high sidelobes. Steerable beams are well suited for tracking moving ground or airborne targets from an airborne radar. SONAR for marine applications is another example where phased array systems are well suited for detecting moving targets. Phased array antennas can include a linear array or a two-dimensional array, be airborne, spaceborne or in the SONAR case under water.
Moving Target Indication (MTI) radars use phased array antennas and measure Doppler shift to detect target velocity. These are usually Pulse Doppler radars, which have advantages over continuous wave (CW) radars because the same antenna elements can switch between transmitting and receiving pulses. By modulating a high frequency carrier with a series of pulses and directing the pulses towards a target, the frequency of the pulses reflected by the target back to the antenna will be shifted, which indicates the relative velocity between the moving radar platform and the target. By dividing the time span between pulses into “range gates,” the distance from the target to the antenna is also determinable.
Either due to lack of antenna calibration or electromagnetic field (EMF) interactions between the phased array antennas and the platform that transports the antennas (e.g. an aircraft for a radar, and a ship or submarine for a SONAR), antenna patterns are distorted and can raise sidelobes (i.e. create sidelobe “clutter”) sufficiently to obscure target data. In other words, a radar beam pointing directly at a target aircraft may not yield a Doppler signal sufficiently discernable due to the elevated sidelobe clutter. It is not sufficient to model the EMF characteristics of an airframe because airframe tolerances are variable and are hard to capture accurately enough for modeling. In addition, Space Time Adaptive Processing (STAP) cannot be used to overcome the uncertainty in the individual antenna element patterns (whether due to a lack of calibration before mounting on the aircraft or due to airframe EMF distortion after mounting) in a radar with an analog beamformer, because in this case not all of the antenna elements are digitized; rather they are combined to form one or just a few beams prior to digitization and conversion into the range-Doppler frequency domain.